1. Field of the Invention
The present invention relates to integrated circuit technology and, in particular, to a constant impedance driver for a high speed interface.
2. Description of Related Art
Off-chip drivers are needed to output data, e.g., an address or a logic result, from one integrated circuit (IC) chip and transfer it to another IC chip. Such data is driven through circuitry in various types of devices, such as an IC package, a printed circuit board, or simply wires or cables. Off-chip drivers act as an interface between the output of one IC chip to become input of another IC chip. The most efficient transfer of data is achieved if one knows the interconnecting wiring and load, and designs the off-chip driver to match that impedance. Variable impedance of a driver will cause reflections and data integrity problems.
Memory card systems frequently present a complex inductive/resistive/capacitive (LRC) load with transmission line reflections resulting from multiple dynamic random access memory (DRAM) slots on a single data trace. One method of coping with signal integrity issues in such a system is to use a low-capacitance open-drain driver. The maximum operating frequency is limited by variations in output driver impedance.
U.S. Pat. No. 5,633,605 to Zimmerman et al. entitled Dynamic Bus with Singular Central Precharge is directed to a dynamic bus system with a central precharge device. In this patent, a precharged data bus either remains high denoting a 1 data state, or is pulled low for a 0 data state by an open-drain driver. An advantage of this architecture is lower pin capacitance due to the absence of a PFET pullup in each output driver. The output stage of this type of driver is shown in FIG. 1, and includes a single NFET device and an 8-ohm ballast resistor with a nominal output impedance of 17-ohms. The impedance of this driver is a strong function of process voltage and temperature (PVT).
In view of the problems of the prior art, there is a need for a constant impedance driver. It would also be advantageous to have compensation for a driver to maintain a predetermined drive impedance across variations in process, voltage and temperature (PVT), as well as providing for adjustment of the constant impedance value.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide a constant impedance driver for data transfer between IC devices.
It is another object of the present invention to provide a method and system for compensating a data driver to maintain a predetermined drive impedance across variations in process, voltage and temperature.
A further object of the invention is to provide a constant impedance driver which is adjustable to different impedances, while still maintaining a constant impedance over a wide voltage range.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects and advantages, which will be apparent to one of skill in the art, are achieved in the present invention which is directed to, in a first aspect, an integrated circuit including a controlled impedance off-chip driver circuit. The integrated circuit comprises an off-chip driver device which controls output impedance of the off-chip driver circuit according to a level of a control signal input thereto, with the off-chip driver being presented with variable operating conditions. The integrated circuit further includes a control circuit which generates the control signal. The control circuit includes a mimic circuit having a sample driver device being a scaled representative of the off-chip driver device. The level of the control signal varies according to the variable operating conditions presented to the sample driver device. A desired output impedance of the off-chip driver circuit is maintained despite variation in the variable operating conditions.
The off-chip driver device and the sample driver device each may include an NFET, and the control signal may include a variable voltage signal provided to the gate of the NFET. In another embodiment, the off-chip driver device and the sample driver device may each include a PFET, and the control signal may include a variable voltage signal provided to the gate of the PFET.
The control circuit may further include a differential amplifier having inputs from the sample driver and a voltage divider circuit, and outputting a variable voltage signal as the control signal to the off-chip driver device.
Preferably, the mimic circuit further includes a constant current source which supplies the sample driver device, and a sample resistance through which the sample driver is supplied by the constant current source. The sample resistance is scaled and representative of resistance in the off-chip driver circuit.
In another aspect, the present invention is directed to a compensated driver for maintaining constant impedance during data transfer from an integrated circuit comprising an output portion having an output device adapted to transfer data from the integrated circuit and a mimic circuit portion having a sample output device scaled to a fraction of the output device adapted to accept a reference current and generate a sample voltage. The driver further includes a differential amplifier portion adapted to generate a control voltage in response to a reference voltage and the sample voltage. There is also provided a means for switching either the control voltage, a power supply voltage or ground to the output stage in response to a data input, the control voltage regulating the output device in the output stage portion to achieve a more constant impedance.
Preferably, the driver further includes a reference portion adapted to generate a reference voltage, more preferably a voltage divider circuit. The output stage portion may be adapted to change to a different, constant impedance in response to a value of the reference current or to a value of the reference voltage. Preferably, the output device of the output stage portion comprises a multiple of the sample output device of the mimic circuit portion.
The driver may further include a constant current source which supplies the reference current, and a sample resistance in the mimic circuit portion through which the sample output device is supplied by the constant current source, wherein the sample resistance scaled and representative of resistance in the output portion.
In its preferred embodiment, the mimic circuit portion and the differential amplifier portion comprise a negative feedback network to generate the control voltage, and the switching means comprises a predrive portion adapted to apply either a ground or the predetermined control voltage from the differential amplifier portion to the output stage portion in response to an input. The output stage portion may include an NFET output device such that the switching means applies either the ground or the predetermined control voltage to the NFET output device. In another embodiment, the output stage portion may include a PFET output device such that the switching means applies either a power supply voltage or the predetermined control voltage to the PFET output device.
In a further aspect, the present invention provides, in an integrated circuit, a method of maintaining a desired output impedance of an off-chip driver circuit having an off-chip driver device despite variations in operating conditions. The method comprises generating a variable level control signal using a sample driver device being a scaled representative of the off-chip driver device. The control signal varies in level according to variable operating conditions presented to the sample driver device, with the variable operating conditions also being presented to the off-chip driver device. The method further includes applying the variable level control signal to the off-chip driver device to control output impedance of the off-chip driver circuit. Desired output impedance of the off-chip driver circuit is maintained despite variation in the variable operating conditions.
The generating step may further comprise comparing a voltage obtained from operating the sample driver device to a reference voltage, and supplying the sample driver device with constant current. Such constant current may be supplied to the sample device through a sample resistance scaled and representative of resistance in the off-chip driver device.
In yet another aspect, the present invention is directed to a method of compensating a driver for maintaining constant impedance during data transfer from an integrated circuit. The method comprises providing an output portion having an output device adapted to transfer data from the integrated circuit, and generating a reference voltage. A reference current is accepted and there is generated a sample voltage in a mimic circuit portion having a sample output device scaled to a fraction of the output device. A control voltage is generated in a differential amplifier portion in response to the reference voltage and the sample voltage. The method also includes switching either the control voltage, a power supply voltage or ground to the output stage in response to a data input, the control voltage regulating the output device in the output stage portion to achieve a more constant impedance.
Preferably, the reference voltage is generated by a voltage divider circuit. The method may further include changing the reference current or the reference voltage to change the output portion to a different, constant impedance. The output device of the output portion preferably comprises a multiple of the sample output device of the mimic circuit portion. In the preferred embodiment, the differential amplifier develops an error voltage and apply it to the control voltage to cause the mimic circuit portion and the differential amplifier portion to reach equilibrium.
The reference current may be a constant current, and the method may further include providing a sample resistance in the mimic circuit portion scaled and representative of resistance in the output portion, such that the constant current flows through the sample resistance.
The output portion may include an NFET output device such that the switching step applies either the ground or the predetermined control voltage to the NFET output device. In another embodiment, the output portion may include a PFET output device such that the switching step applies either a power supply voltage or the predetermined control voltage to the PFET output device.